Merc 1965 / 65 hp lower unit exchange options

I have a 1965 Mercury / 65 HP 650 4 cylinder outboard, can I exchange my original lower unit with a close to the same year lower end but from a 115 HP instead. Reason my shaft is of the older type steel shaft (non pre-load pin type) and is badly pitted / rusted and leaks water past the seal into the lower gear case. The unit from the 115 HP has a much sought after stainless shaft (non pre-load) and looks identical except for the metal shift shaft retainer. The retainer is plastic on my lower end instead of metal as on the 115 HP. Most important, being from a larger power head are the gears ratios different and compatable to my original lower end ? If both lower units are fully inter-changeble then can I switch out my plastic shift shaft retainers only ?? Thanks, MR. Bass Buster.